1. Field of the Invention
The invention relates to a torque sensor that detects a torque exerted on a rotary shaft.
2. Description of Related Art
This kind of conventional torque sensor is described in Japanese Patent Application Publication No. 2008-58026 (JP 2008-58026 A). The torque sensor described in JP 2008-58026 A is configured as a so-called twin resolver-type torque sensor, and includes a first resolver that detects a rotation angle of a first rotary shaft and a second resolver that detects a rotation angle of a second rotary shaft. The first rotary shaft and the second rotary shaft are coupled to each other via a torsion bar.
The first resolver includes a rotor fitted on the outer periphery of the first rotary shaft, and a stator that surrounds the outer periphery of the rotor. The stator is provided with multi-phase output coils. Upon reception of excitation signals, multi-phase signals that vary in accordance with the rotation angle (electric angle) of the rotor are output from the multi-phase output coils of the first resolver. Therefore, the rotation angle (electric angle) of the first rotary shaft is computed based on the multi-phase signals output from the first resolver.
The second resolver has the same configuration as that of the first resolver, except that the shaft angle multiplier is different from that of the first resolver. Note that the shaft angle multiplier indicates a multiplying factor of a voltage signal output from each output coil, in other words, a multiplying factor used to obtain an electrical angle of the rotor from a mechanical angle of the rotor. The rotation angle (electric angle) of the second rotary shaft is computed on the basis of the multi-phase signals output from the second resolver.
In the torque sensor described in JP 2008-58026 A, a rotation angle of the first rotary shaft and a rotation angle of the second rotary shaft are computed by output signals from the respective two resolvers, and then a torsion amount of the torsion bar is obtained by computing a value of the difference between the rotation angles. Finally, a torque exerted on the rotary shaft is computed from the torsion amount.
When the shaft angle multipliers of the two resolvers are different from each other as in the torque sensor described in JP 2008-58026 A, it is necessary, during assembly of the torque sensor onto the rotary shaft, to mount the first resolver onto the first rotary shaft and to mount the second resolver onto the second rotary shaft. However, if a worker erroneously mounts the rotor of the first resolver onto the second rotary shaft and the rotor of the second resolver onto the first rotary shaft, appropriate signals are not output from the two resolvers. As a result, the rotation angles of the two rotary shafts are not appropriately detected, which creates a possibility that a torque will not be accurately detected.